Chemical Equilibrium
Simplifications/assumptions we've been working under:
1. Reactions go to completion ("")
2. Only the forward reaction is involved
3. Reactions stop when completed
Potential Energy
None of these things are strictly true!
What have we learned from kinetics?
Reactants
Products
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Reaction Progress
Chemical Equilibrium
Reasonable to write ALL reactions as reversible.
– Requirements?
Generic Equilibrium:
k1
aA + bB
cC + dD
k-1
At equilibrium rate forward = rate reverse
k1(AA)a(AB)b = k-1(AC)c(AD)d
AA = “activity” of compound (or element) A
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Chemical Equilibrium
Moving all the constants to one side
Activity is a measure of effective reactivity…”opportunity to get
involved” in the process of interest.
AA = A[A] (for solutions) or AA = ApA (for gases)
In equilibrium
q
considerations, we typically
yp
y relate ((ratio)) all conditions to
a “reference state”, co (1 M for solutions, 1 atm for gases)
  C C   D D 
 o   o 
K   c  a c  b
  A A    B B 
 o   o 
 c   c 
c
d
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Equilibrium Constant Expression
Generic Equilibrium Constant Expressions (“kind of”…A):
Kc = [C]c[D]d = [products]x
[A]a[B]b
[[reactants]
t t ]y
Kp = (pC)c(pD)d = pproductsx
( A)a (p
(p
( B) b
preactantsy
NOTE: K is dimensionless!
IMPORTANT! CRITICAL! ESSENTIAL! CRUCIAL!
1. In order for a system (reaction) to be at equilibrium, the Keq
expression MUST be satisfied!!!
2. If the Keq expression is satisfied, the system is in equilibrium!
We can use this to predict the extent of a reaction and/or the
direction a reaction is likely to proceed.
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Equilibrium Constant Expression
Expressing Keq in various concentrations:
• Gases
Describe K in terms of partial pressures
WHY?
aA(g) + bB(g) cC(g) + dD(g)
Kp =
(pC)c(pD)d
(pA)a(pB)b
• Solutions
Describe K in terms of molar concentrations (M)
Often noted at Kc
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Linking Kinetics and Thermodynamics
• Relationship between tendencies and activation parameters
– Thermodynamics and kinetics must cooperate for a reaction to
proceed toward products
>1
Grxn Tendency
Diagram
E
Energy
K k1 ? k-1
Reactants
Reaction
Progress
Products
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Linking Kinetics and Thermodynamics
<1
Reaction
Progress
Products
Energy
Reactants
=1
Grxn Tendency
Diagram
Energy
K k1 ? k-1
Reactants
Reaction
Progress
Products
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Golden Rules of Equilibria
1. Keq is dimensionless and constant at a given temperature.
2. When writing equilibrium constant expressions, omit solids, pure
liquids and solvents
liquids,
solvents.
– WHY?
3. Always use smallest integer coefficients when balancing equations
and writing Keq.
H2 + I2
2 HI
½H2 + ½I2
HI
@ Eq.: [HI] = 1.58 M, [H2] = 0.212 M, [I2] = 0.212 M, calculate Kc.
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4
Golden Rules of Equilibria
4. When a balanced reaction is reversed, Kreverse = 1/Kforward
H2 + I2 2 HI
vs
2 HI H2 + I2
5. When adding reactions, Keq for the net reaction is the product of the
Keq's for the individual reactions.
Example: Calculate the Kp for the gas phase reaction:
H(g) + Br(g) HBr(g)
H2(g) + Br2(g) 2 HBr(g)
Br2(g) 2 Br(g)
H2(g) 2 H(g)
K1 = 7.9 x 1011
K2 = 4.8 x 10-41
K3 = 2.2 x 10-15
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Golden Rules of Equilibria
6. At equilibrium, the concentrations of reactants and products MUST
satisfy the equilibrium constant expression!!
Example: Acetic acid is a weak acid that dissociates via the equilibrium
shown below. We dissolve 1.00 mole of acetic acid in 1.00 L of
water. After allowing the solution to come to equilibrium, the [H3O+]
is measured to be 0.00423 M. If the equilibrium constant for this
process is 1.8 x 10-5, what is the ratio of dissociated to
undissociated acid?
CH3COOH(aq) + H2O(l)
CH3COO-(aq) + H3O+(aq)
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Using Keq to Predict “Direction" of
Reaction
When given a set of conditions
(concentrations) ask:
1. Is the system
y
alreadyy
at equilibrium?
2. If yes, you're done! If no,
what does the system need
to do to reach equilibrium?
Accomplish (1) by calculating Reaction
Quotient (Q): K with nonequilibrium
values Compare Q with Keq
values.
Q = Keq
Q > Keq
Q < Keq
System is in equilibrium
System too product "heavy"
System too reactant "heavy"
Accomplish (2) by "mapping" the reaction and doing a little math.
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Using Keq to Predict “Direction" of
Reaction
Example:
N 2 + O2
2 NO
At 2000K the Kc for the formation of NO is 4.0 x 10-4. You have a
container in which, at 2000 K, the concentration of N2 is 0.50 M, O2 is
0.25 M, and NO is 4.2 x 10-3 M. Is the system at equilibrium? If not,
which direction must the reaction proceed to approach equilibrium?
Calculate the equilibrium concentrations of N2, O2, and NO.
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Le Chatelier's Principle and
Bullying Reactions
Le Chatelier's Principle
Start with system in equilibrium,
equilibrium what happens if we perturb things?
1. Change Concentrations:
2. Change Pressure:
3. Change Temperature:
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